[lldb] [Host] Refactor XML converting getters

Refactor the XML converting attribute and text getters to use LLVM API.
While at it, remove some redundant error and missing XML support
handling, as the called base functions do that anyway.  Add tests
for these methods.

Note that this patch changes the getter behavior to be IMHO more
correct.  In particular:

- negative and overflowing integers are now reported as failures to
  convert, rather than being wrapped over or capped

- digits followed by text are now reported as failures to convert
  to double, rather than their numeric part being converted

Differential Revision: https://reviews.llvm.org/D110410

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